Ultrahigh strength steels

ABSTRACT

An ultrahigh strength material made from ordinary 18-8 stainless steel has a tensile strength in excess of 400,000 p.s.i. The method of producing the ultrahigh strength is accomplished by thermo-mechanical operations. The material can have any desired geometric cross section configuration and is adaptable for use as a spring material.

BACKGROUND OF THE INVENTION Field of the Invention

This invention is in the field of high strength steels and, moreparticularly, in the field of ultra-high strength 18-8 stainless steelmaterials.

Description of the Prior Art

With the invention of adding approximately 18% chromium and 8% nickel toa relatively carbon-free iron, rustless steel or stainless steel wasborn. Since that early development in the 1910's, many modifications andpermutations have been made of the basic 18-8 stainless steel. Later,this basic material was classified as an austenitic stainless steelbecause other classes of stainless steels that were not austenitic werecoming into existence. Even more recently, all the stainless steels havebeen reclassified with this early group of 18-8 stainless steels nowreferred to as AISI type 300 series stainless steels. The basic 18-8stainless steel is now generally referred to as AISI type 302 stainlesssteel.

Type 302 stainless exhibits an austenitic structure and cannot bereadily or appreciably transformed by heat treatment only into asubstantially martensitic structure. However, many attempts have beentried to make type 302 stainless steel stronger and harder primarily byexotic heat treatments without altering the oxidation and corrosionresistance thereof. In retrospect, it was not expected that suchattempts would work and in essence they have not worked. However, type302 stainless steel and special compositions thereof have beenstrengthened by cold work with achievable results indicating strength upto a maximum of 355,000 p.s.i. for standard type 302 stainless and380,000 p.s.i. for special compositions thereof. These specialcompositions constituting small, chemical changes do not significantlyalter the structure of the material.

On the other hand, drastic alterations in chemical compositions havebeen introduced substantially increasing the strength of stainless steelmaterials, yet changing the oxidation and corrosion resistance thereof.In some instances, additional materials were, of necessity, added tothese compositions to restore the oxidation and corrosion resistancecharacteristics. The result of these changes have created new familiesof stainless steels, which are far more expensive and more limited intheir particular scope of usage than the basic type 302 stainless.

The 355,000 p.s.i. to 380,000 p.s.i. strength levels achievable incurrent type 302 stainless steel alloys are far below the strengthlevels achievable in the highest strength alloy steel materials usablefor springs. Some of these alloy steels are used to make fine springs,yet are not conducive for use in oxidizing or other corrosiveatmospheres. Consequently, it has long been recognized that it would beextremely desirable to extend the strength range of type 302 stainlesssteel to provide a good, general high strength stainless steel materialthat could be used for springs, such as in oxidizing or corrosiveatmospheres.

SUMMARY OF THE INVENTION

This invention relates to oxidation and corrosion resistant steels, andis concerned with new and improved characteristics of 18-8 stainlesssteel alloys that provide ultra-high strength levels from over 400,000p.s.i. to over 600,000 p.s.i. This invention also relates to a new andimproved thermo-mechanical method of treating such 18-8 stainless steelsto achieve these ultra-high strength levels.

Briefly, an 18-8 stainless steel material is subjected to a series ofdeformation hardening operations always performed below therecrystallization temperature for the material with intermediateanneals. Subsequently, the material is deformation hardened to a veryhigh level of cold work, over 85%. The material is further processed bybeing subjected to an intermediate heat treatment to inhibit dynamicrecovery, and to obtain an aging response thereby enhancing the strengthalready achieved from the deformation hardening. The material can befurther deformation hardened with intermediate heat treatments to againincrease the strength to higher levels, yet still exhibiting excellentoxidation resistance at temperatures in the 500° F. range. Thus, suchmaterials will provide excellent spring characteristics and oxidationresistance hitherto unknown in 18-8 stainless steel spring materials.

It is an object of this invention to provide an 18-8 stainless steelwith a strength level in excess of 400,000 p.s.i., yet retaining theoxidation and corrosion resistance that is exhibited by this material inits normal cold-worked state.

It is another object of this invention to provide a method forprocessing 18-8 stainless steel to ultra-high strength levels bythermo-mechanical treatment of the material.

It is a feature of the invention to provide such ultra-high strength18-8 stainless steel materials to be used as springs.

It is another feature of this invention to provide an 18-8 stainlesssteel material having an ultra-high strength in cross-sectionalconfigurations as desired, e.g., circles, squares, rectangulars,I-shapes, elongated rectilinear shapes, T-shapes, etc.

Still another feature of the invention is to provide an ultra-highstrength 18-8 stainless steel material in an operating range up to about500° F. yet retaining good oxidation resistance.

Yet another feature of the invention is to provide a plurality of 18-8stainless steel filaments, each having a tensile strength in excess of400,000 p.s.i.

Another feature of the invention is to provide a composite materialhaving the plurality of 18-8 stainless steel filaments surrounded bymetal matrix material, said composite exhibiting a tensile strength inexcess of 400,000 p.s.i.

Another feature of the invention is to provide an 18-8 stainless steelwire having an ultra-high strength with a corrosion resistant coveringsuperior to the core stainless steel.

The above and other and further objects and features of the inventionwill be more readily understood by reference to the following detaileddescription and the accompanying drawing.

DESCRIPTION OF THE DRAWING

FIG. 1 is a cross sectional view of a circular tube surrounding acircular wire;

FIG. 2 is a cross sectional view of a sheathing material having acircular external configuration surrounding a core material having asquare external configuration;

FIG. 3 is a cross sectional view of a sheathing material having acircular external configuration surrounding a core material having arectangular external configuration;

FIG. 4 is a cross sectional view of a sheathing material having acircular external configuration surrounding a core material having anhexagonal external configuration;

FIG. 5 is a cross sectional view of a sheathing material having acircular external configuration surrounding a core material having anI-shaped external configuration;

FIG. 6 is a cross sectional view of a sheathing material having acircular external configuration surrounding a core material having anelongated external configuration;

FIG. 7 is a cross sectional view of the configuration of FIG. 1 whereinthe sheath is drawn down tightly on the core material and having aninterface designated therebetween; and

FIG. 8 is a schematic flow chart of the process utilized in producingthe high strength stainless steel material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a preferred embodiment of this invention, an 18-8 stainless steelcore material having a cross-sectional configuration such as a circle, asquare, a rectangle, a hexagon, an I-shaped, an elongated rectilinearshape, etc., is clad or sheathed with a metal such as a nickel-copperalloy, type 310 stainless steel nickel base superalloys, cobalt basesuperalloys, nickel-cobalt alloys, copper base alloys, lead, titaniumand its alloys, to form a composite. The composite is then reduced incross section so that the sheath tightly adheres to the core. When it isdesired that the distortion of a core shape be held to a minimum, thenthe interior of the sheath should conform to the exterior of the core.The cold deformation may be performed by drawing, swaging, rolling,pressing, squeezing, etc., or any desired combination thereof.

As defined for use in this disclosure, type 302 stainless steel (18-8stainless steel) has the following approximate chemical analysis byweight:

    ______________________________________                                                    Percent                                                           Carbon        .01-.15                                                         Chromium      17-19                                                           Copper         0-.5                                                           Manganese     0-2                                                             Molybdenum     0-.9                                                           Nickel         7-10                                                           Phosphorus     0-.04                                                          Silicon        0-1.5                                                          Sulfur         0-.03                                                          Iron          Balance                                                         ______________________________________                                    

which is substantially the same as the United States Government's AMSSpecification. All constituent elements, except for carbon, that arepresent in less than one percent quantities are considered andcharacterized as minor elements for the purpose of this disclosure. Inaddition, the percent reduction, the percent cold deformed state, thepercent cold worked state, etc., are all the same as the percentreduction in cross sectional area of the material after the last anneal.In other words, a 97.6% cold worked state is the same as a 97.6%reduction in cross sectional area.

It has been found that it is easier to reduce the cross section of thecomposite when the exterior of the sheath is substantially circular,however, other external configurations of the sheath may be used asdesired. The substantially circular exterior cross section 8 of such acomposite 7 configuration is shown in FIG. 1 wherein the core 10 issurrounded by a sheath 12 and the exterior surface 11 of the core 10 issubstantially circular and adjacent the interior surface 13 of thesheath 12. In a similar manner, the cross-sectional compositeconfigurations for a square 10A, a rectangle 10B, a hexagon 10C, anI-shape 10D, and an elongated rectangle 10E, are shown in FIGS. 2through 6. The sheath 12 is reduced in size tightly on the core toprevent any relative movement between the core 10 and the sheath 12 withan interface 15 provided to promote equal reduction of the core 10 andthe sheath 12, as shown in FIG. 7.

The ratio of the sheath material to the core material depends upon thetypes of materials and configuration. Approximately a 5 to 10% reductionin area is required to provide the initial tight mechanical bond betweenthe sheath 12 and the core 10. When the core 10 is made of a materialsuch as type 302 stainless steel, and the sheath is made of the materialsuch as a nickel-copper alloy, the composite is annealed in a heattreating furnace at a rate of approximately two seconds per mil ofdiameter of the composite. The annealing or recrystallizationtemperature must be sufficiently high to provide a complete solutionanneal of the core. For the type 302 stainless and steel nickel-copperalloy composite, 1950° F. is sufficient to provide the solution annealand cause a minor degree of diffusion bonding to occur between the core10 and the sheath 12 at the interface 15 further insuring that therewill be no relative movement between the core and the sheath. Thecomposite is rapidly quenched as it leaves the annealing furnacepreventing carbide precipitation in the microstructure of the stainlesssteel.

The composite is then subjected to a series of cold reducing steps withintermediate anneals wherein the composite's area is reduced at least75% by cold deformation and preferably 85% by cold deformation after thelast anneal. Each of the intermediate anneals is performed above therecrystallization temperature of the core material. However, thistemperature should be kept as low as possible to prevent extensivediffusion between the core and sheathing materials. It is believed thatthe extreme amount of cold deformation is enhanced because the sheathingsupports the core material, provides a protective coating and functionsas a lubricant. This particular step in the process is extremelyimportant, and heretofore has not been recognized as one of the primarysteps necessary to provide the ultra-high strength material havingcomplicated geometric cross sectional configurations. It is believedthat for simple geometric shapes, such as an elongated rectangle, asquare, a circle, etc., that the cladded sheath is not required,however, it can be used, if desired.

At this stage in the processing, three different series of operationsmay be employed, depending on the desired final strength of the corematerial.

In one preferred embodiment, the 75-84% cold deformed composite isheated for approximately four hours (with the permissable time rangingfrom about one-half hour to about sixteen hours or more) at atemperature well below the lowest recrystallization or transformationtemperature of the core material; for type 302 stainless material, therange would be about 700° F. to 825° F., and preferably in a narrowerrange of about 775° F. to 800° F. For ease in understanding and as usedhereinafter, the sub-transformation temperature of the core materialrefers to a temperature at which substantially no recrystallization ofthe microstructure will occur. This sub-transformation temperature isalso called a stress relieving temperature. Subsequently, the 84% colddeformed composite is additionally cold deformed to as much as 97%,wherein the core material has a tensile strength in excess of 500,000p.s.i. If desired, the sheath can be removed from the core such as bychemical dissolution and other methods well known in the art. When thesheath is a nickel-copper alloy, chemical dissolution in nitric acid isquite satisfactory. If the sub-transformation heat treatment is omitted,then the material at a 97% cold deformed state would exhibit a tensilestrength in excess of 400,000 p.s.i.

In another preferred embodiment of the invention, the 84% cold deformedcomposite is further cold deformed to approximately 93% to 94%. Thecomposite is heat treated at the sub-transformation temperature range of700° to 825° F. for an approximate period of time such as four hours.The composite is subsequently cold deformed to 98% and again heattreated at the sub-transformation temperature range of 700° F. to 825°F. for an approximate period of time, such as four hours. The sheath canbe removed as described above, with the core having a resulting tensilestrength ranging from approximately 500,000 p.s.i. to approximately580,000 p.s.i.

In still another embodiment of the invention, the 84% cold deformedcomposite was further drawn to a 97.6% cold deformation state. Thecomposite is heat-treated at the sub-transformation temperature of about700° F. to 825° F. for approximately four hours. The composite is thenadditionally cold deformed to a 98.7% state. The core material exhibiteda tensile strength of about 575,000 p.s.i. to 600,000 p.s.i. Thiscomposite is then heat-treated a second time at a sub-transformationtemperature, the same as above, for approxiamtely about 41/2 hours. Thecore material then exhibited a tensile strength in excess of 600,000p.s.i.

In another embodiment of the invention, the 84% cold deformed compositeis further cold deformed to approximately 97% or more. The composite isthen heat treated at the sub-tranformation temperature ranging from 700°F. to 825° F. After the sheath is removed similar to the mannerdescribed above, the core has a resultant tensile strength varying from475,000 p.s.i. to 525,000 p.s.i.

In another embodiment of the invention it is possible to form aplurality of ultra-high strength metal filaments bu substituting thecomposite as taught herein for the wire-sheath composite structure ofU.S. Pat. No. 3,277,564, U.S. Pat. No. 3,394,213, and/or U.S.application Ser. No. 6,709, all owned by the assignee hereof. Theteachings of both of these patents are fully incorporated by referenceherein, and are adaptable for use in forming a plurality of ultra-highstrength stainless steel filaments in accordance with the combinedteachings thereof. Depending on the final application, it is notnecessary to remove the matrix, thereby the end product being acomposite of ultra-high strength filaments of any desired configurationsurrounded by a metal matrix.

The following examples of specific ultra-high strength steels made inaccordance with this invention should not be construed in any way tolimit the scope contemplated by this invention.

EXAMPLE I

A type of 302 stainless steel rod having a 0.080 inch diameter and anapproximate chemical analysis by weight of:

    ______________________________________                                                    Percent                                                           Carbon         .09                                                            Silicon       1.23                                                            Manganese     1.14                                                            Phosphorus    .021                                                            Sulfur        .010                                                            Chromium      16.9                                                            Nickel        8.00                                                            Molybdenum     .7                                                             Nitrogen      .045                                                            Iron          Balance                                                         ______________________________________                                    

and was surrounded by a Monel K sheath having a 0.115 inch outsidediameter, 0.100 inch inside diameter, and a chemical analysis of nickel,66%; copper, 29%; and aluminum, 3%. The rod-sheath composite was drawnthrough a 0.091 inch diameter wire drawing die. The composite wassolution annealed at 1950° F. at a rate of two seconds per mil ofdiameter of the composite and rapidly quenched. The composite was colddrawn through a series of dies with intermediate anneals to a 97.6% coldworked state. The composite was then heat treated at asub-transformation temperature of about 795° F. for approxiately fourhours. The sheathing material was removed and the resulting stainlesswire exhibited a tensile strength of 540,100 p.s.i.

EXAMPLE II

Same as Example I except that after the sub-transformation heattreatment of the composite, the composite was further cold drawn fromthe 97.6% state to a 98.7% state of cold work. The sheathing materialwas then stripped from the composite and the resultant stainless steelrod exhibited a tensile strength of 592,000 p.s.i. Prior to removing thesheathing the tensile strength was 472,800 p.s.i. with the Monel actingas corrosion resistance coating.

EXAMPLE III

Same as Example II except that a final heat treatment at asub-transformation temperature of 795° F. for approximately 41/2 hourswas employed. The sheathing material was then removed from thecomposite, and the stainless steel wire exhibited a tensile strength of608,000 pounds per square inch.

EXAMPLE IV

A type 302 stainless steel rod having a 0.080 diameter and anapproximate chemical analysis by weight of:

    ______________________________________                                                    Percent                                                           Carbon         .09                                                            Silicon       1.23                                                            Manganese     1.14                                                            Phosphorus    .021                                                            Sulfur        .010                                                            Chromium      16.9                                                            Nickel        8.00                                                            Molybdenum     .7                                                             Nitrogen      .015                                                            Iron          Balance                                                         ______________________________________                                    

and was surrounded by a Monel K sheath having a 0.115 inch outsidediameter, 0.100 inch inside diameter, and a chemical analysis of nickel,66%; copper, 29%; and aluminum, 3%. The rod-sheath composite was drawnthrough a 0.091 inch diameter wire drawing die. The composite wassolution annealed at 1950° F. at a rate of two seconds per mil ofdiameter of the composite and rapidly quenched. The composite was colddrawn through a series of dies with intermediate anneals prior toachieving a 99.4% cold worked state. The composite was then stripped ofits sheathing material and exhibited a tensile strength of 535,500p.s.i.

EXAMPLE V

The same as Example IV except that after the sub-transformation heattreatment of the composite, the composite was further cold worked from a99.4% cold work state to a 99.6% cold work state. The sheathing materialwas removed therefrom, with the final stainless steel wire exhibiting atensile strength of 619,000 pounds per square inch.

EXAMPLE VI

Ninety-one (91) type 302 stainless steel rods, each having a diameter of0.080 inch were placed in Monel 400 tubes, each having a 0.115 inchoutside diameter and a .100 inch inside diameter. The chemicalcomposition of the 302 stainless steel by weight was:

    ______________________________________                                                    Percent                                                           Carbon         .10                                                            Silicon        .46                                                            Manganese      .5                                                             Chromium      18.9                                                            Nickel         8.9                                                            Phosphorus    .018                                                            Sulfur        .008                                                            Iron          Balance                                                         ______________________________________                                    

The rod-like combinations were packed in a mild steel billet, heated,evacuated to about 10⁻ ⁵ torr and sealed. The billet was heated andextruded at 1800° F. with a 16 times reduction forming a composite. Thecomposite was then reduced by cold reduction with intermediate anneals.The composite was fully annealed at a rate of two seconds per mildiameter of the composite. The composite was then cold drawn to a finaldiameter of 16.8 mils with each of the individual rods (now filaments)having an effective cross section diameter of about 1.13 mils. Thefilaments were at a 93.8% cold worked state. The strength of the 302stainless steel filaments was found to be 393,200 p.s.i. The compositewas then heat treated at a sub-transformation temperature of about 700°F. for about 16 hours. The final strength of the 302 stainless steelfilaments was then found to be 427,500 p.s.i.

EXAMPLE VII

Same as Example VI except that the composite was cold drawn to a 98.5%cold worked state. The final composite diameter was 16.8 mils with eachof the individual filaments having an effective cross section dimensionof about 1.13 mils. The strength of the 302 stainless filaments wasfound to be 453,700 p.s.i. The composite was then heat treated at asub-transformation temperature of about 700° F. for approximately 16hours. The final strength of the filaments was found to be 512,200p.s.i.

It has been found that during the different variations in processingthat the austenite that originally existed in the core materialtransformed into at least 50% martensite by a diffusionless phasetransformation. The sheathing material is preselected to have a colddeformation rate that is compatible with the cold deformation rate ofthe core material. It is believed that the additional treatment at thesub-transformation temperature range further inhibits dynamic recoveryand obtains an aging response thereby enhancing the strength alreadyachieved from the deformation hardening. After the sheathing is removed,the ultra-high strength core material may be subjected to a final sizingoperation to obtain a uniform cross sectional geometry.

The general processing steps or operations are graphically depicted inFIG. 8. The ordinate denotes the heat treatment range in temperature andthe abscissa depicts the cycle of operations or steps. Operations A andC indicate cold deforming operations with B indicating an intermediateanneal. Operations indicated as A, B and C are size reducing operationsand can be repeated any number of times. D indicates the solution annealof the core material. E indicates the amount of cold work reductionmeasured in prcentages. F indicates the first sub-transformation annealwith G indicating subsequent cold reduction. Lastly, H indicates thefinal sub-transformation anneal.

It can readily be seen that the ultra-high strength materials can becoiled for use in springs, either as tension or compression springs. Inaddition, spiral type watch springs, leaf springs, etc., can also bemade from this material exhibiting such high tensile strength.

Although specific embodiments of the invention have been described, manymodifications and changes may be made, especially in configurations,without departing from the spirit and scope of the invention as definedin the appended claims.

We claim:
 1. A high strength stainless steel material having acomposition by weight consisting essentially of .15% maximum carbon,1.5% maximum silicon, 2% maximum manganese, about 17% to about 19%chromium, about 7to about 10% nickel, minor amounts of other metals, andthe balance constituent iron, and characterized in that said materialhas a tensile strength in excess of 400,000 p.s.i.
 2. The material ofclaim 1 wherein said material exhibits approximately not more than a 20%decrease in strength up to approximately 500° F.
 3. The material ofclaim 1 wherein the material has a preselectedly sized configuration. 4.The material of claim 1 which is formed into a spring.
 5. The materialof claim 1 further including a tightly adhering sheath material.
 6. Thematerial of claim 5 which is formed into a spring.
 7. A method ofobtaining a tensile strength of at least 400,000 p.s.i. from an 18-8stainless steel material closely controlling the steps comprising:(1)rapidly quenching a solution annealed 18-8 stainless steel material toprevent carbide precipitation, (2) cold deforming and material to atleast a 75% cold worked state; and (3) heat treating said cold deformedmaterial at a sub-transformation temperature to inhibit dynamic recoveryand provide an increase in strength.
 8. The method of claim 7 furtherincluding the step of cold deforming said material subsequent to saidheat treating.
 9. The method of claim 7 wherein the sub-transformationheat treatment temperature ranges from 775° F. to 800° F.
 10. The methodof claim 7 wherein the material is further worked to an 84% cold workedstate during cold deforming.
 11. The method of claim 7 wherein thematerial is further worked to a 95% cold work state during colddeforming.
 12. The method of claim 7 wherein the material is furthercold worked to at least a 97.6% cold work state during cold deforming.13. The method of claim 12 wherein after the sub-transformationtemperature heat treatment the material is further cold worked to atleast a 98.7% cold worked state with the material exhibiting a tensilestrength in excess of 575,000 p.s.i.
 14. A method of obtaining a tensilestrength of at least 400,000 p.s.i. from an 18-8 stainless steelmaterial comprising the steps of:(1) rapidly quenching a solutionannealed 18-8 stainless steel material to prevent carbide precipitation;and (2) cold deforming said material to at least a 97% cold workedstate. .Iadd.
 15. A process for thermo-mechanically treating 18-8stainless steel, comprising the steps of: (a) cold deforming the 18-8steel, (b) heating the steel to a temperature in the sub-transformationtemperature range of 700°F to 825°F to prevent carbide precipitation,and (c) cooling the steel to a temperature below about 700°F and thenagain cold deforming the steel, the 18-8 steel totally being deformed aminimum of 75%. .Iaddend. .Iadd.
 16. A process for thermo-mechanicallytreating 18-8 stainless steel, comprising the steps of: (a) heating thesteel to a temperature in the recrystallization temperature range toanneal said steel, (b) cooling the steel to a temperature range of 700°Fto 825°F and then cold deforming said steel, (c) heating the steel to atemperature in the sub-transformation temperature range, and (d) coolingthe steel to a temperature below about 700°F and then again colddeforming the steel, the 18-8 steel totally being cold deformed aminimum of 75%. .Iaddend. .Iadd.17. The process of claim 15 wherein thetotal amount of deformation is in excess of 75%..Iaddend.